I really appreciate having a high-level diagram of system architecture when joining a new project. It gives me an instant feel for whats going on and even if its slightly out of date it gives context to the evolution and design decisions of the codebase since then. That said, there's nothing that beats the good ol' step debugger and diving into the code first hand. They're complementary. Without the diagram its kind of like building a mental model of a room by feeling it out in the dark.

Edit: I'm not too strict on UML exactness, as long as it gives a feel for the components, data flow, etc.